1. Technical Field
The present invention relates to a switch circuit.
2. Related Art
Japanese Laid-open patent publications No.H11-74703 (patent document 1) and No.H09-93001 (patent document 2) disclose switch circuits for use under a millimeter-wave band (30 to 300 GHz) that include a field-effect transistor (hereinafter, FET) serving as a switching element. In the switch circuits, the FET appears to be an ON resistance between the source and the drain when the channel is open, and can be handled as an OFF capacitance between the source and the drain when pinched off. The switch circuit according to the patent document 1 is a high-pass type switch circuit designed based on characteristics of a high-pass filter. On the other hand, the switch circuit according to the patent document 2 utilizes a LC serial resonance of an inductor and the OFF capacitance of the FET.
FIG. 15 is a circuit diagram of the switch circuit according to the patent document 1. The switch circuit includes FETs 103, 104 in a path connecting input/output (hereinafter, I/O) terminals 101 and 102. The FETs 103, 104 are serially connected to each other, so that the source of either is connected to the drain of the other. To a connection point A between the FETs 103, 104, an end of an inductor 105 is connected, with the other end grounded. In the switch circuit thus configured, ON and OFF are switched upon applying a common voltage to the respective gate of the FETs 103, 104, through a resistor 106.
When the FETs 103, 104 are pinched off, a circuit constituted of the respective OFF capacitance of the FETs 103, 104 and the inductor 105 becomes identical to an equivalent circuit of a T-type high-pass filter, as shown in FIG. 16. Under such state, accordingly, the switch circuit of FIG. 15 presents a low loss characteristic in a frequency range not lower than the cutoff frequency, and the switch is turned ON. In contrast, when the channels of the FETs 103, 104 are open, the impedance of the circuit formed by the ON resistance of the FETs 103, 104 provokes a matching loss, and the switch is turned OFF.
FIG. 17 is a circuit diagram of the switch circuit according to the patent document 2. The switch circuit includes transmission lines 113 to 115 serially connected to one another, in a path connecting I/O terminals 111, 112. Between a connection point of the transmission lines 113, 114 and the ground, two paths are provided. One of the paths includes a FET 116, and the other path includes a FET 117 and a transmission line 118. Likewise, between a connection point of the transmission lines 114, 115 and the ground, a path including a FET 119 and a path including a FET 120 and a transmission line 121 are provided. The gates of the FETs 116, 119 are mutually connected, so that between a connection point thereof and a bias terminal 122, a transmission line 123 is provided. Likewise, the gates of the FETs 117, 120 are mutually connected so that between a connection point thereof and a bias terminal 124, a transmission line 125 is provided.
Here, the characteristic impedance of the transmission lines 113, 114, 115, 118, 121, 123 and 125 is 50Ω. The length of the transmission lines 123, 125 is equal to a quarter of a wavelength (hereinafter, λ/4), at an operating frequency.
The switch circuit thus configured is turned ON and OFF by switching the open channel state and the pinched-off state of the shunted FETs 116, 119 and the FETs 117, 120. When the FETs 116, 119 are pinched off and the channels of the FETs 117, 120 are open, the equivalent circuit can be expressed as FIG. 18A. As is apparent from FIG. 18A, the shunt circuit gains high impedance because of the LC parallel resonance, and the switch is turned ON. When the states of the FETs 116, 119 and the FETs 117, 120 are reversed, the equivalent circuit turns to what is shown in FIG. 18B. As is apparent from FIG. 18B, because of the LC serial resonance of the transmission lines 118, 121 acting as the inductor and the OFF capacitance of the FETs 116, 119, the shunt circuit becomes short-circuited, and the switch is turned OFF.